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Facial expressions have been extensively used to assess emotions in humans and thus could be extended to other species that also display facial movements. In mice both painful and fearful situations have been associated with particular shifts in facial expressions. Like other species, mice frequently show a great inter-individual variability when exposed to emotional situations, but so far no study has been conducted to investigate if facial expressions are related to these differences. The aim of this study is to explore if mice of wild origin (Mus spicilegus) express different facial expressions when confronted to novelty and to relate these mimics to their emotional reactivity profile. We used individual exploration scores in a novel odour test and in the elevated plus maze test as proxy measures of individual emotional reactivity. Our results showed that exploration scores in both tests were positively correlated, and both were related to the ear postures expressed by the individuals during their first exploration of the novel odour. This single component of facial expression was in fact a good indicator of inter-individual differences expressed in these two different tests suggesting a strong link between this marker and the individual emotional reactivity. These results highlight the great potential of facial expressions to assess emotional states in animals. Copyright © 2015. Published by Elsevier B.V.
Content may be subject to copyright.
Behavioural
Processes
120
(2015)
25–29
Contents
lists
available
at
ScienceDirect
Behavioural
Processes
jo
ur
nal
homep
ag
e:
www.elsevier.com/locate/behavproc
Correlates
between
ear
postures
and
emotional
reactivity
in
a
wild
type
mouse
species
Benjamin
Lecorps,
Christophe
Féron
Laboratoire
d’Ethologie
Expérimentale
et
Comparée,
Université
Paris
13,
Sorbonne
Paris
Cité,
Avenue
JB
Clément,
Villetaneuse
93430,
France
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
19
March
2015
Received
in
revised
form
5
August
2015
Accepted
7
August
2015
Available
online
12
August
2015
Keywords:
Facial
expressions
Anxiety
Emotions
Elevated
plus
maze
Inter-individual
differences
a
b
s
t
r
a
c
t
Facial
expressions
have
been
extensively
used
to
assess
emotions
in
humans
and
thus
could
be
extended
to
other
species
that
also
display
facial
movements.
In
mice
both
painful
and
fearful
situations
have
been
associated
with
particular
shifts
in
facial
expressions.
Like
other
species,
mice
frequently
show
a
great
inter-individual
variability
when
exposed
to
emotional
situations,
but
so
far
no
study
has
been
conducted
to
investigate
if
facial
expressions
are
related
to
these
differences.
The
aim
of
this
study
is
to
explore
if
mice
of
wild
origin
(Mus
spicilegus)
express
different
facial
expressions
when
confronted
to
novelty
and
to
relate
these
mimics
to
their
emotional
reactivity
profile.
We
used
individual
exploration
scores
in
a
novel
odour
test
and
in
the
elevated
plus
maze
test
as
proxy
measures
of
individual
emotional
reactivity.
Our
results
showed
that
exploration
scores
in
both
tests
were
positively
correlated,
and
both
were
related
to
the
ear
postures
expressed
by
the
individuals
during
their
first
exploration
of
the
novel
odour.
This
single
component
of
facial
expression
was
in
fact
a
good
indicator
of
inter-individual
differences
expressed
in
these
two
different
tests
suggesting
a
strong
link
between
this
marker
and
the
individual
emotional
reactivity.
These
results
highlight
the
great
potential
of
facial
expressions
to
assess
emotional
states
in
animals.
©
2015
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
In
a
population,
individuals
greatly
and
consistently
differ
in
their
response
to
stressors
(Wilson,
1998)
leading
them
to
cope
differently
with
challenging
situations.
Individual
differ-
ences
became
recently
an
ebullient
field
of
research
leading
to
the
definition
of
different
notions
such
as
personality
traits
(Wilson
et
al.,
1994),
behavioural
reactions
norms
(Dingemanse
et
al.,
2010)
or
behavioural
syndromes
(Sih
et
al.,
2004)
in
verte-
brates
and
invertebrates
(Gosling,
2001;
Kralj-Fiˇ
ser
and
Schuett,
2014).
Many
traits
related
to
affective
states
have
been
studied
such
as
aggressiveness,
boldness
or
neophobia,
mostly
through
behaviours
expressed
in
response
to
environmental
and
social
stressors.
The
persistent
behavioural
and
physiological
response
to
these
stressors
has
notably
led
to
the
concept
of
coping
styles
describing
two
main
profiles
of
individuals:
the
proactive
indi-
viduals
(bolder
and
more
aggressive
individuals)
expressing
more
active
behavioural
strategies
in
challenging
situations
associated
with
higher
activation
of
the
sympathetic
adrenomedullary
system
(SAM);
and
the
reactive
individuals
who
react
more
passively
and
Corresponding
author.
Tel.:
+33
0632112536.
E-mail
address:
benjamin.lecorps@gmail.com
(B.
Lecorps).
expressed
a
stronger
hypothalamic–pituitary–adrenocortical
axis
(HPA)
activity
(Koolhaas
et
al.,
1999).
Coping
styles
would
therefore
involve
differences
in
both
behavioural
strategies
and
physiologi-
cal
response
(both
SAM
and
HPA
axis
are
key
endocrine
systems
sustaining
the
emotional
response
of
animals)
reflecting
an
indi-
vidual
and
consistent
way
to
cope
with
different
stressful
situations
(Koolhaas
et
al.,
2011).
In
the
past
decades
efforts
have
been
made
to
establish
a
link
between
specific
markers,
especially
behavioural
ones
and
animals’
emotional
states
(Boissy
et
al.,
2007;
Désiré
et
al.,
2002).
However,
emotions
in
animals
remain
highly
difficult
to
access
and
to
infer
due
to
specificities
in
emotional
signalling
and
to
the
lack
of
verbal
language
(Grandin
and
Deesing,
2013).
Studies
on
individual
vari-
ation
highlight
the
strong
and
consistent
behavioural
differences
that
exist
among
individuals,
and
such
research
would
be
greatly
complemented
by
efficient
markers
of
animals’
emotional
states.
While
continuities
and
analogies
between
facial
expressions
of
animals
and
humans
have
already
been
mentioned
by
Darwin
(1872),
until
recently
few
studies
have
focused
on
facial
expres-
sions
in
non-primate
animals,
which
also
exhibit
facial
movements
(Diogo
et
al.,
2009).
However,
a
recent
increase
of
interest
has
led
to
studies
carried
out
in
many
species.
For
instance,
facial
expressions
have
been
linked
to
emotional
states
of
dogs
(Bloom
and
Friedman,
2013)
and
farm
animals
(Boissy
et
al.,
2011;
Schmied
et
al.,
2008;
http://dx.doi.org/10.1016/j.beproc.2015.08.002
0376-6357/©
2015
Elsevier
B.V.
All
rights
reserved.
26
B.
Lecorps,
C.
Féron
/
Behavioural
Processes
120
(2015)
25–29
Waring,
2003;
Reefmann
et
al.,
2009;
Costa
et
al.,
2014;
Proctor
and
Carder,
2014).
A
particular
emphasis
has
been
put
on
the
ear
position
of
these
animals,
which
is
one
of
the
most
mobile
parts
of
the
face
in
many
mammals
and
thus
relatively
easy
to
assess.
For
example,
pain
faces
have
been
described
in
horses
(Gleerup
et
al.,
2014),
cats
(Holden
et
al.,
2014)
and
rabbits
(Keating
et
al.,
2012).
In
laboratory
rodents,
while
facial
expressions
have
been
suc-
cessfully
used
to
assess
pain
(Langford
et
al.,
2010;
Sotocinal
et
al.,
2011)
and
have
been
also
studied
in
fear-related
contexts
(Defensor
et
al.,
2012),
their
study
is
still
in
its
early
stages
(Makowska
and
Weary,
2013).
These
markers
could
be
particularly
useful
in
rodent
species
as
their
emotions
are
of
matter
of
interest
in
the
fields
of
psychopharmacology
and
behavioural
neurosciences.
Besides,
con-
trary
to
larger
animals,
physiological
measures
reflecting
emotional
responses
and
anxiety
levels
in
real-time
(e.g.
heart
rate,
bodily
temperature)
are
still
difficult
in
small
mammals
such
as
rodents,
especially
due
to
the
need
of
invasive
procedures
(i.e.,
surgical
pro-
cedures).
This
lack
can
make
difficult
the
assessment
of
emotions
in
those
species
and
could
be
somehow
counterbalanced
by
the
use
of
new
and
more
efficient
behavioural
markers
of
emotional
states
such
as
facial
expressions.
Therefore
with
the
perspective
of
becoming
a
useful
tool
to
assess
emotions
in
rodents,
facial
expres-
sions
still
need
to
be
characterized
in
diverse
emotional
situations
and
contexts.
“Exploration
is
driven
by
an
approach/avoidance
conflict”
(Augustsson
and
Meyerson,
2004)
and
is
frequently
used
as
a
proxy
measure
of
emotional
reactivity
of
animals
(Ohl,
2003).
Exploration
of
novel,
unfamiliar
and
stressful
environments
has
been
exten-
sively
used
to
assess
emotional
states
in
rodents
and
especially
their
anxiety
levels
(File,
2001;
Ohl,
2003).
This
has
been
mostly
done
by
assessing
components
of
the
general
exploration
pattern
(Ohl,
2003).
Even
in
very
simple
contexts
mice
could
express
a
continu-
ity
of
response
from
the
more
reactive
to
the
more
proactive
ones
depending
on
their
general
emotional
reactivity
profile.
Therefore,
if
facial
expressions
are
somehow
related
to
the
emotional
response
of
the
animals,
these
mimics
should
be
different
from
one
animal
to
the
other,
mainly
depending
on
how
an
individual
perceives
the
situation
and
thus
be
related
to
behaviours
expressed
during
the
tests.
In
this
study,
we
used
a
standard
elevated
plus
maze
(EPM)
test
and
a
modified
novel
object
test,
introduced
by
the
use
of
a
novel
odour.
The
EPM
test
showed
a
remarkable
efficiency
as
a
post-hoc
test
to
assess
anxiety
levels
of
animals
(Carobrez
and
Bertoglio,
2005)
and
to
discriminate
individuals
towards
their
emo-
tional
reactivity
on
the
basis
of
open
arms
exploration
(Trullas
and
Skolnick,
1993;
Holmes
et
al.,
2000).
Here,
its
purpose
was
to
help
differentiating
individuals
towards
their
coping
styles
(proac-
tive
vs.
reactive)
or
emotional
reactivity.
Besides,
we
presented
a
novel
odour
instead
of
a
more
classical
novel
object
as
it
has
been
shown
to
induce
comparable
effects
in
rodents
(i.e.,
stress
response)
(Kemble
and
Bolwahnn,
1997).
But,
exposure
to
novel
odour
instead
of
novel
object
better
fulfils
the
purpose
of
this
study
as
the
intro-
duction
of
an
odour
can
be
done
less
invasively
compared
to
the
use
of
an
object,
which
could
disrupt
proper
video
recordings
of
facial
expressions
during
testing.
We
expected
that
behaviours
expressed
in
these
two
tests
would
give
larger
insights
in
individual
differences
in
exploration
pat-
terns
and
thus
in
emotional
reactivity
of
animals.
The
novel
odour
(NO)
test
was
particularly
designed
to
facilitate
facial
expression
recording
in
a
highly
standardized
way
(mice
were
all
in
the
same
position).
We
aimed
to
test
the
animals
during
their
activ-
ity
period
(in
condition
of
high
reactivity).
Facial
expressions
were
thus
recorded
during
the
dark
period.
However,
these
recording
conditions
compelled
us
to
limit
our
analysis
to
the
most
obvious
facial
expressions:
the
ear
position.
A
species
of
wild
origin,
the
mound
building
mouse
(Mus
spi-
cilegus)
was
used
in
this
study,
as
wild
type
species
have
been
shown
to
express
more
inter-individual
differences
and
com-
paratively
stronger
emotional
reactivity
compared
to
laboratory
animals
(Augustsson
and
Meyerson,
2004;
Koolhaas
et
al.,
2001;
Price
and
Grandin,
1998).
2.
Material
and
methods
2.1.
Animals
Nineteen
sexually
naïve
adult
males
M.
spicilegus
were
used
in
this
study.
We
only
focused
on
males
as
females’
oestrous
cycle,
already
known
to
impact
emotional
reactivity
(Morgan
and
Pfaff,
2002)
would
have
complicated
the
analysis.
Animals
were
kept
under
laboratory
conditions
under
a
14:10
light:dark
cycle
and
an
ambient
temperature
of
20 C
(±2).
Food
and
water
were
always
provided
ad
libitum.
2.2.
Novel
odour
test
The
NO
test
took
place
in
two
connected
Plexiglas
boxes
(Fig.
1).
We
habituated
the
animals
to
this
environment
for
5
days
before
testing,
the
effect
of
novelty
was
therefore
only
due
to
the
odour
itself.
All
the
tests
were
carried
out
during
the
activity
period
of
ani-
mals
(dark
period).
At
the
test
day,
1
ml
of
a
non-repellent
odour
(liquid
with
an
anise
aroma)
(Colombelli-Negrel
and
Gouat,
2006)
mixed
with
wood
shavings
was
introduced
in
a
perforated
stainless
steel
tube
placed
in
a
corner
of
the
odour
compartment.
The
analy-
sis
focused
on
the
first
approach
of
the
animal.
We
used
the
latency
to
reach
the
odour
(since
they
entered
in
the
odour
compartment),
the
time
spent
at
distance
(animals
were
at
least
at
20
cm
away
from
the
odour),
the
time
spent
close
to
the
odour
before
contac-
ting
it,
and
the
distance
covered
by
animals
during
their
approach
(since
they
entered
in
the
odour
compartment)
to
assess
the
ani-
mals’
general
approach
strategy.
Ethovision
XT
7
(Noldus)
was
used
for
behavioural
analysis.
2.3.
Facial
expressions
Screen
shots
were
extracted
from
videos
selecting
those
when
the
animal
approached
and
contacted
the
odour-dispensing
device
for
the
first
time.
Considering
that
approach
ear
postures
were
not
Fig.
1.
Two-compartment
apparatus
used
for
the
novel
odour
test.
Identical
Plex-
iglass
boxes
(36
×
24
cm)
were
connected
by
a
4
cm
hole.
The
nest
compartment
included
water,
food
and
nest
materials
(cotton
balls).
The
odour
compartment
included
a
stainless
steel
tube
to
silently
introduce
the
odour.
B.
Lecorps,
C.
Féron
/
Behavioural
Processes
120
(2015)
25–29
27
Fig.
2.
Scale
used
for
ear
posture
assessment.
Following
a
clear
and
short
explanation
of
the
scoring
rules,
pictures
were
presented
to
observers
in
a
random
order.
A
discrete
scale
between
2
(backward
position)
to
+2
(forward
position)
was
used.
Pictures
presented
are
examples
of
pictures
scored
by
the
observers.
available
for
all
animals
and
the
good
consistencies
between
the
two
postures
rated,
we
decided
to
only
focus
on
facial
expressions
expressed
at
the
first
contact
with
the
odour
device.
Six
inexperi-
enced
observers
were
asked
to
rate
ear
position.
A
discrete
scale
ranging
between
2
(backward
position)
to
+2
(forward
position)
with
3
intermediary
values
was
used
to
describe
the
posture
of
the
ears
(Fig.
2).
Medians
of
the
6
scores
per
individuals
were
used
for
statistical
analyses.
2.4.
Elevated
plus
maze
test
EPM
tests
were
performed
5
days
after
the
NO
test.
The
EPM
consisted
of
4
arms,
two
open
arms
of
30
cm
and
two
closed
arms
of
30
cm
with
walls
of
30
cm
high
and
was
elevated
50
cm
above
the
floor
level.
We
used
the
percentage
of
time
spent
in
the
open
arms
and
of
open
arms
entries,
the
time
spent
freezing,
and
the
total
number
of
arms
entries
to
describe
both
activity
and
anx-
iety
patterns
(File,
2001;
Lafaille
and
Féron,
2014).
Furthermore,
anxiety-related
behaviour
in
the
elevated
plus
maze
has
been
shown
to
be
repeatable
across
consecutive
tests
in
this
species
(Rangassamy
et
al.,
2015).
2.5.
Statistical
analyses
All
analyses
were
done
using
the
software
R
version
2.14.2
(R
Core
Team,
2013).
Principal
component
analyses
(PCA)
were
used
to
assess
the
general
exploration
scores
of
individuals
in
both
tests
using
the
FactoMineR
package.
Thereby
coordinates
of
animals
on
the
first
axis
of
both
PCA
(see
electronic
appendix)
were
used
to
assess
relationships
between
exploration
scores
expressed
in
both
tests
and
between
exploration
scores
and
ear
postures.
All
correla-
tions
were
tested
with
a
non-parametric
correlation
test
(Spearman
rank
test).
3.
Results
The
scale
used
for
ear
posture
assessment
displayed
high
inter-rater
reliability
assessed
by
intraclass
correlation
coefficient
(ICC
=
0.82)
(Shrout
and
Fleiss,
1979).
In
the
NO
test,
the
first
axis
of
PCA
(describing
59%
of
the
variabil-
ity)
discriminated
individuals
towards
their
approach
strategy:
fast
and
direct
(low
latencies
and
short
distance
covered
before
reach-
ing
the
odour)
vs.
slow
and
indirect
(high
latencies,
more
distance
covered
and
more
time
at
distance)
(see
electronic
appendix).
In
the
EPM,
the
first
axis
of
PCA
(representing
61%
of
the
variability)
well
described
the
classical
distinction
between
anxious/inactive
and
bold/active
profiles
(more
time
in
open
arms,
less
time
freezing
and
more
arms
entries)
(see
electronic
appendix).
Fig.
3.
Correlation
between
exploration
scores
in
the
EPM
and
the
NO
test.
Explo-
ration
scores
have
been
defined
using
the
first
axis
of
each
principal
component
analysis
(see
text
for
justification
and
statistics).
As
expected,
general
exploration
scores
in
both
tests
were
positively
correlated
(Spearman
rank:
rs=
0.600,
P
=
0.007;
Fig.
3).
Individuals
described
as
more
anxious
in
the
elevated
plus
maze
showed
a
faster
and
more
direct
approach
to
the
novel
odour.
Ear
postures
were
negatively
correlated
with
exploration
scores
in
the
NO
test
(rs=
0.740,
P
<
0.001).
Slow
and
indirect
approaches
to
the
odour
were
related
with
ears
in
a
backward
position
whereas
fast
and
direct
approaches
were
related
with
ears
in
a
forward
position
(Fig.
4A).
A
consistent
negative
correlation
was
also
found
between
ear
postures
in
the
NO
test
and
exploration
scores
in
the
EPM
(rs=
0.610,
P
=
0.005;
Fig.
4B).
4.
Discussion
The
aim
of
this
study
was
to
investigate
whether
the
facial
mim-
ics
expressed
by
mice
when
exploring
a
novel
odour
are
related
to
their
emotional
responsiveness.
Focusing
our
attention
on
ear
posi-
tion,
a
component
of
facial
mimics
that
can
be
easily
and
clearly
rated,
even
under
red
light
condition,
our
goal
was
then
to
test
if
this
parameter
will
prove
its
usefulness
to
assess
emotional
responses
in
mice.
We
used
exploration
scores
in
both
tests
as
proxy
measures
for
emotional
reactivity.
As
expected,
exploration
scores
in
the
two
tests
were
strongly
and
positively
correlated,
that
is
the
most
reac-
tive
individuals
in
the
EPM
(less
time
in
open
arms)
were
those
who
showed
the
most
direct
and
fastest
approach
towards
the
novel
odour.
Such
direct
approaches
of
subjects
to
a
novel
stimu-
lus
(generally
a
novel
object)
have
been
controversially
discussed.
Mostly
seen
as
a
sign
of
neophillia
(Ennaceur
et
al.,
2009;
Lafaille
28
B.
Lecorps,
C.
Féron
/
Behavioural
Processes
120
(2015)
25–29
Fig.
4.
(A)
Correlations
between
exploration
scores
in
the
NO
test
and
ear
position
(median
of
rating
scores)
and
(B)
between
exploration
scores
in
the
EPM
and
ear
position.
Exploration
scores
have
been
defined
using
the
first
axis
of
both
PCA
(see
text
for
justification
and
statistics).
and
Féron,
2014),
direct
approaches
are
also
interpreted
as
anti-
predator
strategy
more
likely
to
be
associated
with
fear
(Grandin
and
Deesing,
2013).
Although
being
exposed
to
a
novel
odour
has
been
described
to
be
stressful
whatever
the
nature
of
the
odour
(Kemble
and
Bolwahnn,
1997),
we
suggest
that
the
exposure
to
a
novel
non-social
odour
would
probably
be
less
stressful
than
to
be
confronted
to
a
novel
object.
Furthermore,
the
most
proactive
indi-
viduals
are
often
described
as
being
less
attentive
to
small
changes
in
their
environment
(Grandin
and
Deesing,
2013;
Koolhaas
et
al.,
2010).
This
would
explain
a
reduced
interest
towards
the
novel
odour
compared
to
more
reactive
individuals,
as
observed
in
our
study.
Therefore,
in
the
context
of
novel
odour
exposure,
the
high
latencies
could
be
seen
rather
as
a
sign
of
disinterest
than
of
anxiety,
and
the
low
latencies
as
an
indication
of
emotional
reactivity.
Ear
position
showed
high
inter-rater
reliability,
even
under
red-
light
conditions.
Furthermore,
we
found
high
consistencies
when
assessed
during
the
animals’
approach
to
the
odour
device
and
during
the
first
contact
(data
not
shown).
Our
results
clearly
indicate
a
relationship
between
exploration
scores
in
both
tests
and
ear
postures
of
animals
in
the
NO
test.
Ears
displayed
in
the
forward
position
were
both
linked
to
fast
and
direct
approaches
to
the
novel
odour
and
to
a
lower
exploration
of
the
open
arms
of
the
EPM.
The
exploration
pattern
of
each
individual
was
thus
well-linked
to
the
ear
position
expressed
when
contacting
the
novel
odour.
To
our
knowledge
this
is
the
first
time
that
facial
expressions
in
a
rodent
are
shown
to
be
linked
to
emotional
reactivity,
and
espe-
cially
to
anxiety-related
behaviours
expressed
in
a
well-established
unconditioned
fear
test.
Based
on
our
findings,
we
conclude
that
forward
ear
posture
will
be
predominantly
expressed
by
animals
with
a
more
reactive/anxious
profile
when
approaching
a
novel
stimulus.
Ear
expressed
forward
is
an
unambiguous
posture,
which
can
be
easily
rated
by
naïve
observers.
Only
26%
of
animals
displayed
an
ear
posture
scored
strictly
superior
to
0
(rated
“forward”)
and
up
to
42%
when
scores
of
0
are
added.
These
intermediate
values
(0
values)
showed
a
strongest
variability,
which
could
be
related
with
difficulties
to
be
assessed
by
the
naïve
observers
(comments
not
shown).
The
majority
of
animals
were
scored
as
having
ears
in
the
backward
position
(58%).
With
few
exceptions,
the
animals
display-
ing
this
posture
adopted
a
quite
similar
exploration
profile
in
both
tests.
This
might
probably
reflect
a
“ground”
effect,
as
we
suppose
that
the
novel
odour
test
was
not
really
anxiogenic
for
most
indi-
viduals.
We
cannot
exclude
that
the
use
of
a
more
invasive
stressor
would
induce
less
variability
in
ear
posture.
But
we
can
infer
due
to
our
results
that
in
this
kind
of
situation,
the
animals
should
mostly
express
ears
in
the
forward
position.
According
to
recent
unpub-
lished
results,
wild-type
Mus
domesticus
predominantly
adopt
such
an
ear
position
when
exposed
to
a
predator
odour.
This
facial
expression
could
serve
a
risk
assessment
function,
as
a
focal
acoustic
and
visual
attention
directed
towards
the
odour
obviously
permits
the
animal
to
at
least
check
whether
the
new
stimulus
is
associated
with
movements
or
sounds.
Ears
in
forward
position
have
been
also
found
in
other
species
when
a
particu-
lar
attention
is
required,
as
exemplified
by
studies
in
dogs
(Racca
et
al.,
2012),
horses
(Waring,
2003)
and
sheep
(Boissy
et
al.,
2011;
Reefmann
et
al.,
2009).
This
suggests
that
ears
oriented
forward
are
a
part
of
a
vigilance
posture
shared
by
many
mammals
in
stressful
situations.
The
relationship
we
found
between
facial
expressions
and
the
way
animals
behave
in
the
elevated
plus
maze
test,
also
sus-
tains
the
fact
that
ear
posture
is
clearly
related
to
inter-individual
differences
in
emotional
reactivity
and
supports
previous
evidence
that
facial
expressions
could
be
extended
to
other
non-primate
mammalian
species
for
the
assessment
of
emotional
states.
5.
Conclusion
The
results
of
this
study
suggest
that
the
position
of
the
ears
dis-
played
by
mice
when
exploring
a
novel
non-social
odour
depended
on
how
the
subjects
perceived
this
new
stimulus.
Thus,
we
con-
clude
that
ear
position
could
be
considered
as
a
risk
assessment
posture
and
as
a
new
and
promising
marker
of
emotional
reac-
tivity
in
mice.
Further
investigations
are
nevertheless
required
to
clarify
the
association
between
facial
expressions
and
emotions
in
rodents.
Ethical
statement
This
study
was
registered
as
Ce5/2012/111
and
approved
by
the
Ethics
Committee
in
Animal
Experiment
Charles
Darwin.
Conflict
of
interest
statement
The
authors
declare
no
conflict
of
interest.
Acknowledgements
The
authors
wish
to
thank
L.
Jaravel
for
excellent
animal
care
and
H.G.
Rödel
for
helpful
discussions
and
polishing
the
English.
B.
Lecorps,
C.
Féron
/
Behavioural
Processes
120
(2015)
25–29
29
Appendix
A.
Supplementary
data
Supplementary
data
associated
with
this
article
can
be
found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.beproc.2015.
08.002.
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